Charged particle beam system and method of operating a charged particle beam system

ABSTRACT

The present disclosure relates to a gas field ion source having a gun housing, an electrically conductive gun can base attached to the gun housing, an inner tube mounted to the gun can base, the inner tube being made of an electrically isolating ceramic, an electrically conductive tip attached to the inner tube, an outer tube mounted to the gun can base, the outer tube being made of an electrically isolating ceramic, and an extractor electrode attached to the outer tube. The extractor electrode can have an opening for the passage of ions generated in proximity to the electrically conductive tip.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e)(1) to U.S.Provisional Application Nos. 61/843,772, 61/843,777, 61/843,779,61/843,785 and 61/843,812, filed Jul. 8, 2013. The contents of theseapplication are hereby incorporated by reference in their entirety.

FIELD

This disclosure relates to a charged particle beam system, a chargedparticle source for a charged particle beam system, especially a gasfield ion source, and methods of operating a charged particle beamsystem.

BACKGROUND

Charged particle source, charged particle systems and methods ofoperating charged particle systems and sources can be used for variousapplications including measuring or identifying sample properties or forsample modification. A charged particle source typically produces a beamof charged particles that can be directed by components of a chargedparticle beam system to be incident on a sample. By detectinginteraction products of the charged particle beam with the sample imagesof a sample can be generated or properties of the sample can beidentified.

The following documents include prior art which can be of some relevancefor the present disclosure: EP2088613A1, EP2182542A1, US2012119086,EP2068343A1, EP2110843A1, US2012132802, US2012199758, WO 2007067310,WO08152132A2.

SUMMARY

According to a first aspect the disclosure relates to a gas field ionsource, comprising a gun housing, an electrically conductive gun canbase attached to the gun housing, an inner tube mounted to the gun canbase, the inner tube being made of an electrically isolating material,

an electrically conductive tip attached to the inner tube, an outer tubemounted to the gun can base, the outer tube being made of anelectrically isolating material, and an extractor electrode attached tothe outer tube. The extractor electrode can have an opening for thepassage of ions generated in proximity to the electrically conductivetip.

The electrically isolating material of the inner tube as well as theelectrically isolating material of the outer tube can be an electricallyisolating ceramic.

According to an embodiment the gas field ion source further comprises agas supply having a terminating tube attached to the gun can base.

According to a further embodiment the gas field ion source furthercomprises a thermal conductor, for example cold braids, connected to guncan base, the thermal conductor being thermally connected to a coolingdevice.

According to a still further embodiment of the gas field ion source thecooling device can be a dewar.

According to a still further embodiment the gas field ion source furthercomprises a heater within the inner tube and electrically connected tothe electrically conductive tip.

According to a still further embodiment the gas field ion source furthercomprises a flapper valve arranged at the gun can base to increase a gasflow from a region within the outer tube into a region surrounding theouter tube.

According to a still further embodiment of the gas field ion source avolume enclosed by the gun can base, the outer tube and the extractorelectrode is smaller than 60 cm³.

According to a still further embodiment the gas field ion source furthercomprises a first high voltage supply electrically connected to theelectrically conductive tip and a second high voltage sourceelectrically connected to the extractor electrode. The first highvoltage source and the second high voltage source also can be realizedby a single high voltage source, for example a DC to DC voltageconverter, providing different high voltages on its output side.

According to a still further embodiment the gas supply can be configuredto supply a first gas in a first mode of operation and a second gas in asecond mode of operation, and wherein the first gas and the second gasare different gases.

According to a still further embodiment the gas field ion source canfurther comprise a vacuum pump functionally connected to the outerhousing to evacuate gas out of the outer housing.

According to a still further embodiment the gas field ion source canfurther comprise a heat shield configured to reduce radiative heattransfer from the gun housing to a volume surrounded by the outer tube.

DESCRIPTION OF DRAWINGS

Details of embodiments will hereinafter be described with reference tothe attached drawings.

FIG. 1 shows the mechanical set-up of a charged particle beam system ina sectional view.

FIG. 2 shows an enlarged sectional view of the particle chamber of thecharged particle beam system in FIG. 1.

FIG. 3 shows an enlarge sectional view of gas field ion source.

FIG. 4 shows a sketch of the charged particle beam system including avacuum system.

FIG. 5 shows a three dimensional representation of an outer housing of agas field ion source.

FIG. 6 shows a flow chart showing various steps in cleaning processes ofa gas field ion source operated with a noble gas such as neon.

FIG. 7 shows a flow chart showing the adjustment of a tip apex.

FIG. 8 shows a motorized leak valve

FIG. 9 shows a flow diagram regarding the adjustment of the air flow fora gas field ion source.

FIG. 10 shows a principle sketch of a gas field ion beam system with anelectrical set-up.

FIGS. 11 a and 11 b show images of a gas field ion source emitter tip.

FIG. 12 show a cross section of a gas field ion source with a heatshield.

FIGS. 13 a and 13 b show embodiments of heat shields allowing gasexchange.

DETAILED DESCRIPTION

The charged particle beam system 1 in FIG. 1 comprises a sample chamber10 which is positioned and mounted on a heavy and massive table 5. Thetable 5 can be a granite plate or a plate made of concrete. The table 5itself rests on a number of first legs 3 a and 3 b of which two areshown in FIG. 1. The first legs 3 a, 3 b are designed to becomepositioned on a floor 2. Each of the first legs 3 a, 3 b comprises orsupports a first vibration isolation member 4 a, 4 b to avoid thetransmission of vibrations from the floor to the table 5.

The sample chamber 10 rests on the table 5 via a number of second legs18 a, 18 b also each comprising or supporting a second vibrationisolation member 9 a, 9 b. These second vibration isolation members 9 a,9 b serve to reduce or avoid the transmission of vibrations from thetable 5 to the sample chamber. Such vibrations of the table 5 canoriginate from a mechanical vacuum pump 17, for example a turbo pump,which is firmly attached to or mounted on the table 5. Due to the largemass of the table 5 the vibration amplitudes generated by the mechanicalpump 17 are greatly reduced.

The mechanical pump 17 is functionally connected to the sample chamber10. For this functional connection a suction port of pump 17 isconnected via two flexible bellow portions 6, 8 with a stiff tube 7 or acompact vacuum flange between both flexible bellow portions to thesample chamber 10. The complete line from the pump 17 to the samplechamber forms a series arrangement of “flexible bellow portion—stifftube—flexible bellow portion”. This arrangement serves to furtherattenuate the vibrational energy, and reduce the vibrations transmittedfrom the table to the chamber. The vibrations of the chamber can bereduced further when the mass of the intermediate tube is large. Thevibrations of the chamber can be further reduced if there is an energyabsorbing material in contact with the bellows or the tube. The chambervibrations can be further reduced if there is a mechanical resonance ofthe tube and bellows that preferentially absorbs and dissipates thevibrational energy at the frequencies caused by the pump 17.

In a particular embodiment described later, the charged particle beamsystem can have more than one mechanical pumps, especially two turbomolecular pumps. As a ways of reducing the effect of vibrations on theimage quality, both (or if there are more than two turbo molecular pumpsall) of the turbo-molecular pumps are connected to the charged particlebeam system via a sequential pair of flexible bellows. The chargedparticle beam system can have two turbo-molecular pumps, one for thechamber and one for the gun.

The double bellow arrangement serves to prevent the vibrations inherentin the rotational frequency of the pump (e.g. 900 Hz or 1 kHz and theirharmonics) from transferring to the microscope. The turbo pump itself ismounted firmly to the large granite platform (table 5) selected for itslarge mass and inherent damping capabilities, and this serves todiminish the measurable vibration on the granite to the nanometer orsub-nanometer level. The vibration transfer to the microscope is furtherreduced by the sequence of two successive bellows with a stiff tubebetween them. The turbo vibration measured in the sample chamber 10 orin the region of the charged particle source is generally belowsub-nanometer or sub-angstrom level. In this manner, adequate pumpingspeed can be attained (e.g. 200 liters/second of vacuum pumping speed ormore) without adversely degrading image quality.

The sample chamber 10 has a vacuum tight housing 19. A tubular extension11 is firmly and non-detachable mounted to the housing 19 of the samplechamber 10. The tubular extension 11 can be formed by a metal tubewelded to the remaining portions of the housing 19 surrounding thesample chamber 10. Alternatively, the tubular extension can be anintegral part of the chamber housing itself.

Within the tubular extension 11 a charged particle column 12 is mounted.The charged particle column 12 thereby comprises lenses, diaphragms andbeam scanning systems not shown in FIG. 1. By directly mounting thecomponents of the charged particle column 12 within a tubular extensionof the housing 19 of the sample chamber 10 mechanical vibrations betweenthe components of the charged particle column and a sample stage 20arranged within the sample chamber 10 can be avoided or at leastreduced.

On the tubular extension 11 of the housing 19 of the sample chamber amodule comprising the charged particle source is attached. This modulecomprises a lower housing portion 16 having an upper spherical surfacewhich forms one portion of a two axes tilt mount. In addition thissource module comprises an upper housing 15 in which the chargedparticle emitter is mounted. In the shown case the charged particlesource is a gas field ion source and the charged particle emitter 14 isan electrically conductive tip. The upper housing 15 also has aspherical surface portion forming a second part of the two axis tiltmount. By the aid of this tilt mount the upper housing portion 15holding the charged particle emitter 14 can be tilted about two axesrelative to the charged particle column 12 to align the axis of emissionof charged particles emitted by the charged particle emitter 14 to anoptical axis defined by the charged particle components arranged withinthe charged particle column 12.

The tilt mount can be designed as an air bearing in the manner thateither the spherical surface of the upper housing 15 or the sphericalsurface of the lower housing 16 comprises small channels (not shown)through which an air flow can be provided which lifts the upper housingso that the upper housing is easily moveable relative to the lowerhousing. By stopping the air flow the upper housing and the lowerhousing are held together by strong frictional forces between the upperhousing and the lower housing.

In FIG. 2 the housing 19 of the sample chamber 10 with the tubularextension is shown in more details. The charged particle column 12mounted in the tubular portion 11 comprises several diaphragms 22, adeflection system 23 and an objective lens 21 with which a chargedparticle beam can be focused onto a sample and scanned across a samplewhich can be positioned on a sample stage (not shown here) in the samplechamber 10. In the case that the charged particle beam system is a gasfield ion beam system the lens 21 and the deflection system 23 areelectrostatic components which act on the ions by electrostatic forcesdue to different electrostatic potentials applied to components of thesystems. In addition the charged particle column 12 comprises a firstpressure limiting aperture 24 and a second pressure limiting aperture 25which form an intermediate vacuum region (mid column region 70) betweenthe vacuum region in which the emitter tip 14 is positioned and thesample chamber 10. The component of the charged particle column 12closest to the emitter of the gas field ion source is an electrode 26forming a part of a condenser lens followed by a deflector 27 foraligning the beam coming from the gas field ion source to the opticalaxis defined by the charged particle optical components followingdownward in the direction of beam propagation to the sample chamber 10.

In FIG. 3 the design of a compact gas field ion source is shown. Thisgas field ion source is designed with double nested insulators. This isa compact design while still providing high voltage, variable beamenergy, and gas containment. The design consists of several parts. Thefirst part is a thermally conductive (e.g. copper) base platform 31which is grounded and is directly linked and thermally connected to acryogenic cooling system 52 by way of a flexible thermal conductor 32,for example copper ribbons or copper braids. The base platformhereinafter as well as above sometimes also is called gun can base. Theflexibility of the thermal conductor 32 allows the complete gas fieldions source to tilt, and to minimize any vibration transport. Thethermal conductivity of the braids allows them to also heat the gasfield ion source as a periodic maintenance procedure.

The cryogenic cooling system can be a dewar 52 filled with liquid and/orsolid nitrogen. Alternatively, the cryogenic cooling system can be adewar filled with solid nitrogen. The dewar can comprise a heater 73 cwith which the dewar as well as the base platform 31 can be heated.Alternatively, the cryogenic cooling system can be a mechanicalrefrigerator.

Attached to this grounded base platform 31 is a central tubular highvoltage insulator 33, for example made of alumina or sapphire, thatmechanically supports the electrical conductive tip 34 which forms thegas field ion emitter. The central tubular insulator 33 provides over 30kV of electrical isolation with respect to the base platform 31. Thiscentral insulator 33 has one or more openings for connection of highvoltage leads 35, 36 connected to the conductive tip 34 for providingthe high voltage for operating the tip 34 as a gas field ion source andalso to supply a heating current for heating the tip 34.

Also attached to the base platform 31 is an outer tubular andcylindrical insulator 37 that surrounds the central insulator 33. Theouter tubular insulator 37 mechanically supports an extractor electrode38 and provides also more than (over) 30 kV of electrical insulation.

The extractor electrode 38 is designed with a small hole 39 (e.g. 1 mm,3 mm, 5 mm diameter) that by design is a small distance (e.g. 1 mm, 3mm, 5 mm) from the apex of the tip 34. Together the base platform 31,the central insulator 33, the outer cylindrical insulator 37 and theextractor electrode 38 define an inner gas confining vessel 41. Thevacuum conduction or pumping speed through the hole 39 of the extractorelectrode can be relatively small to support a relative high pressure inthe region of the electrically conductive tip 34 compared to the regionoutside the inner gas confining vessel 41. The only passages for gas toescape are the aforementioned extractor hole, and a gas delivery path40, and a pumping valve 42. The gas delivery path is through a smalltube 40 that passes from a supply bottle to the interior gas confiningvessel 41 through the grounded base platform 31. The pumping valve 42can be mounted on the base platform 31, or integrated into the gasdelivery path 40.

All of the above mentioned components of the charged particle source aresupported on the base platform 31 that is mechanically supported by astiff yet thermally non-conductive support structure (not shown) thatmounts to the upper portion of the exterior vacuum vessel (15 in FIG.1). The upper portion of the exterior vacuum vessel is allowed to tiltto a small angle up to 5 degrees by the interface of a concave sphericalsurface with a corresponding convex spherical surface in the lowerexternal vacuum housing (16 in FIG. 1).

Within the inner gas confining vessel an ion getter 45 is arranged.Improved vacuum in the inner gas confining vessel is attained with theinclusion of chemical getters 45 in the interior of the gas confiningvessel 41. These chemical getters 45 are activated at the time of bakingthe gas field ion source. A heater 73 b is provided to heat the chemicalgetters 45. During the heating of the chemical getters 45 to atemperature of about 200° C. for 2 hours and upon cooling thesecomponents the chemical getters 45 leave many chemically activematerials, such as Zr, V, Fe and Ti, etc., that serve to effectivelypump many spurious gas species. The getters can be coated directly ontothe surfaces of existing parts, for example the outer cylindricalisolator 37, or they can be ribbon like materials that are attached tothe interior surfaces forming the inner gas confining vessel. Thepumping speed of the chemical getters for hydrogen is of importancesince among the likely impurities, hydrogen is not effectivelycryo-pumped by the surfaces cooled to cryogenic temperature. Thesechemical getters 45 in the inner gas confining vessel 41 are also veryeffective for further purification of the delivered helium and neongases. Being noble gases, the helium and neon are not affected, but allimpurities will be effectively pumped. During their periodicregeneration, the evolved gases can be pumped away in an improved mannerby opening the purpose-made bypass valve 42 (flapper valve) thatconnects the inner gas confining vessel 41 to the exterior gascontainment 81.

The inner gas confining vessel 41 can be surrounded by a radiationshield which minimizes the radiative heat transfer from the exteriorvessel walls (at room temperature) to the ion source. The inner gasconfining vessel 41 also can contain an optically transparent windowthat allows a direct line of sight onto the tip 34 of the emitter fromoutside the inner vacuum vessel. An aligned window in the exteriorvacuum vessel allows a camera or pyrometer to observe the emitter tip ofthe gas field ion source. Such a camera can inspect the source, ormonitor its temperature during the periodic maintenance. One or both ofthese windows can include leaded glass to minimize radiation transfer ofX-rays from the interior to the exterior. The base platform 31 due toits high thermal conductivity is also well suited for a temperaturesensor such as a thermocouple.

As further described later in more detail, the gas supply tube 40 cancomprise a heater 73 a.

The gas field ion source is operated at a voltage that is establishedbased upon the geometrical shape of the emitter tip, 34. The geometricalshape includes factors such as the average cone angle, and the averageradius of curvature of the emitter tip 34.

The above mentioned design has the advantage of a small mass, and asmall volume. These both allow for faster thermal cycling and reducedcooling load, and reduced cost, and reduced complexity. In addition, thecompact design allows a quick change of the noble gas with which the gasfield ion source is operated. Especially the compact design of the innergas confining vessel 41 allows a quick change between operating the gasfield ion source with helium and operating the gas field ion source withneon.

Under ideal operation conditions, the apex of the emitter tip 34 isroughly spherical (e.g. with a diameter of 50, 100, or 200 nm.) Thespherical surface is in fact better described as a series of planarfacets that approximate a sphere. Near the apex of the tip 34 of theemitter, the end form is better approximated by three planar facets thatintersect at a single vertex forming a three sided pyramid. The pyramidedges can be relatively shallow angled (e.g. 70 or 80 degrees withrespect to the axis of the emitter). The ridges and the apex of thepyramid are somewhat rounded at the atomic level so that there are nosingle atom ridges or that there is not a single atom at the apex.

Under ideal operation conditions there are three atoms of the emittermaterial at the apex which form an equilateral triangle. These threeatoms, hereinafter called the “trimer”, protrude the most, and henceproduce the largest electric field when a positive voltage (e.g. 20 kV,30 kV, 40 kV) relative to the extractor electrode is applied to the tip.In the presence of helium or neon gas, the neutral atoms can be fieldionized just above these three atoms. At relatively high gas pressures(at local pressures of 10⁻² Torr, or 10⁻³ Torr) the ionizations canhappen at rates of 10⁶ or 10⁷ or 10⁸ ions per second. Under the idealcircumstances, this steady stream of ions is constant over time andpersists indefinitely.

To the extent that in this specification the unit Torr is used it can besubstituted by mbar.

In reality, under typical conditions when operating with helium, the ionemission can represent 100 pA of emitted current, and it can persist for10 or more continuous days, and show up and down fluctuations that areon the order of 0.5% over timescales of ms or faster. Gradual loss ofemission current can progress at a rate of 10% per day if uncorrected.The helium performance (or the performance of operation with helium) issomewhat impacted by the purity of the gas which can be 99.9990%, or99.9999% purity or even better and the quality of the base vacuum inabsence of helium typical is 2×10⁻⁹ Torr, 1×10⁻⁹ Torr, 5×10⁻¹⁰ Torr oreven better.

When the gas field ion source is operated with Neon there are severalcomplications compared to the situation when the gas field ion source isoperated with helium. In part, the neon ions are much more massive andhence able to cause sputtering at a rate that can be 50 times more thanhelium. As the neon ions strike nearby surfaces, the sputtered atoms canbe negatively charged (e.g. negative secondary ions) and as such theycan be back accelerated to the emitter 34 and cause the emitter 34 to bedamaged. In part, neon gas is not commercially available (e.g. incompressed gas bottles) with the same levels of purity in which heliumgas is commercially available (e.g. 99.9999% pure). The effect of theseimpurities is discussed later. But most significantly, when the emitter34 is operated with neon, the emitter 34 should be operated at asomewhat reduced voltage. For example, if 40 kV is optimal for helium,the same emitter tip will give an optimal emission current of neon at 30kV. At this reduced voltage, the electric field is similarly reduced,and spurious atoms (residual gases from the imperfect vacuum, orimpurities in the gas supply) are able to reach the emitter 34 at a muchhigher rate. The mere 25% reduction in field strength seems to allow anexponentially larger number of these spurious atoms (not helium and notneon) to reach the emitter. These spurious atoms (e.g. H₂, N₂, O₂, CO,CO₂, H₂O, etc.) can disturb the availability of neon to reach the tip ofthe gas field ion source, and hence causes emission instability both onthe short and on the long time scales. The spurious atoms can alsofacilitate the etching of the emitter material causing it to graduallychange its shape over time which can reduce the ion emission currentgradually and can reduce the optimal operating voltage gradually. Thespurious atoms can also cause one or more of the atoms of the emittertip 34 to be more readily field evaporated causing abrupt emissiondrops.

In order to produce a stable neon beam, or a beam of noble gas ionshaving atoms of a mass larger than neon, the composition of theextractor electrode is quite important. Especially the surface thatfaces the emitter is important. The tip 34 of the gas field ion sourceis configured to be quite close to an adjacent extractor electrode 38with a small hole 39 in it. The tip of the ion source 34 and theextractor electrode 38 have voltages applied to them. The difference involtages gives rise to an electric field which is quite large near theapex of the emitter tip 34. The composition of the extractor electrode38 is made of a material that is not appreciably sputtered with the neonbeam and does not form negative ions, for example carbon, iron,molybdenum, titanium, vanadium, tantalum. Also, the composition of thesurface of the extractor electrode 38 facing the tip 34 is made of amaterial that is readily cleanable, and with a low outgassing rate forultra high vacuum (e.g. stainless steel or oxygen free copper). Thesurface can also have a smoothness (realized by mechanically polishingor electropolishing) to produce a mirror like finish, especially thesurfaces that are nearest to the tip 34 of the emitter. Also thematerial of the surface of the extractor electrode 38 facing the emittertip 34 can have a very low negative secondary ion sputter yield (e.g.gold and other materials free of oxides, nickel). The low sputter yieldfor negative secondary ions reduces the frequency with which negativesecondary ions are created which can be accelerated back to the emitterto cause damaging it (or to cause damaging impacts). The secondaryelectron yield can be as low of 10⁻⁵ per incident neon ion.

In order to produce a stable neon beam, the exact shape of the extractorelectrode 38 is very critical for several reasons. In particular theshape of the extractor hole 39 is critical. There are several designcriteria for the hole in the extractor and the optimal shape is abalance of several conflicting needs. First, to confine the ionizing gas(helium or neon), the hole should be relatively small so that the noblegas is maintained at a relatively high pressure in the range between10⁻² torr and 10⁻³ torr in the inner gas confining vessel 41 near theapex of the emitter tip 34, and allow the pressure to drop significantlyinto the range between 10⁻⁵ torr and 10⁻⁷ torr outside the inner gasconfining vessel 41. Reducing the pressure outside the inner gasconfining vessel 41 is critical to minimize the rate at which thedesirable high energy ions scatter off of the low energy neutral gasatoms. The scattering can give rise to undesirable beam tails, and evenallow some ions to become neutralized. It is thus the vacuum conductanceof the hole 39 in the extractor electrode 38 that is critical. Vacuumconductance is measured in liter per second and is a standard measurefor determining how the pressure falls off from one side of a hole 39(the interior) to the other side of a hole 39 (the exterior).

Also, if the hole 39 in the extractor electrode 38 is too large, theemitter tip 34 of the gas field ion source will be radiatively exposedto warmer surfaces. The emitter tip 34 of the gas field ions source andthe extractor electrode 38 are maintained at cryogenic temperatures inthe range between −210° C. and −190° C. If the hole 39 in the extractorelectrode 38 is too large then the emitter tip 34 of the gas field ionsource will become warmed-up by larger surface areas that are notcryogenically cold, but are instead at room temperature (e.g. +20° C.).Generally, the cryogenically cold surfaces are effectively trappingspurious gas atoms, and warm surfaces do not effectively trap gas atoms.

There are however contrary reasons for which it is desirable that thehole 39 in the extractor electrode 38 not be too small. For example, ifthe hole 39 is too small it becomes a challenge to manufacture and keepthis hole clean to the levels to support the high vacuum and highelectric fields in which it functions. Also, the tip 34 of the emittershould be centered relative to the hole 39 in the extractor electrode 38within 10% of the diameter of the extractor hole 39. Thus, if the holeis too small, it becomes difficult to position it in a symmetric fashionwith respect to the tip 34 of the emitter.

Also, the angular size of the extractor hole cannot be too small withrespect to the apex of the tip 34 of the ion emitter. Expressed anotherway, the solid angle of the extractor hole 39 as seen by the tip 34 ofthe emitter is a certain size. This stems from the pattern of the ionemission. The tip 34 of the emitter itself tends to emit ions with afairly narrow cone, with a half cone angle of 2 degrees. However, it iscommon to have extraneous emission at significantly larger angles. Thenature of the shape of the emitter is that high extraneous emission at a20 degree angle relative to the axis of the emitter is quite common. Andit is desirable that these emitted ions do not strike the extractorelectrode 38 to avoid damage to the extractor electrode 38, or theproduction of negative secondary ions that would damage the tip 34 ofthe emitter. Also, the extraneous ion emission could serve to desorb anyadsorbates that might transfer to the emitter and cause unstable ionemission from the tip of the emitter. Thus, the angular size of the hole39 in the extractor electrode 38 and the distance between the tip 34 ofthe emitter and the extractor electrode 38 is selected so that the angleof the hole is about or greater than 20 degrees in angle.

It is recognized to be of some importance that the interior gasconfining vessel 41 has a very good base vacuum, or equivalently, isvery free from spurious adatoms (e.g. the atoms and molecules other thanthe desired operating noble gas such as helium or neon), Such spuriousatoms and molecules can be H₂, N₂, H₂O, O₂, CO, NO, CO₂, etc. By way ofexplanation, the pressure of the base vacuum in the inner gas confiningvessel 41 is the pressure that would be measured within the inner gasconfining vessel when the supply of gases to the inner gas confiningvessel, especially the supply of helium and neon gases, are turned off.The desired pressure would be 10⁻¹⁰ torr or better. At a pressure of4×10⁻¹⁰ torr, while the background gas pressure is low, it would takeabout 1 hour for an initially clean surface to be covered by spuriousadatoms to a thickness of one monolayer. Such adatoms cause instabilityof the ion source. Thus, it is intended to attain the best possible basevacuum. Towards this end, the entire housing of the gas field ion sourceis configured to be cleaned according to Ultra High Vacuum (UHV)practices. And the interior gas confining vessel 41, where the tip 34 ofthe gas field ion source is housed, is configured and prepared for UHVservice.

FIG. 4 shows the principles of a gas field ion microscope that can beoperated with two different noble gases for the ion beam, in thisparticular case either with helium or with neon. The gas field ionmicroscope has three vacuum regions, within the microscope's housing 19.The first vacuum region is the sample chamber 10, the second vacuumregion is the mid column region 70 and the third vacuum region is theouter vacuum containment 81 in which the gas field ion source is housed.The mid column region 70 is positioned between the outer gas containment81 and the sample chamber 10.

As described before, the sample chamber is evacuated by a turbomolecular pump 17 which is mounted on table 5 (not shown in FIG. 4). Theouter gas containment 81 also is evacuated by a mechanical pump 60 whichalso can be a turbo molecular pump which also can be mounted on table 5.The connection between the mechanical pump 60 evacuating the outer gascontainment 81 can be designed like the connection between pump 17 andthe sample chamber, i.e. the connection between pump 60 and the outergas containment 81 also can comprise two flexible bellows with a stifftube or a compact vacuum flange between them.

The mid column region 70 is separated from the outer gas containment 81by a first pressure limiting aperture 54. In a similar manner the midcolumn region 70 is separated from the sample chamber 10 by a secondpressure limiting aperture 55. The mid column region 70 is evacuated byan ion getter pump 56. This provides the advantage that ion getter pump56 does not generate any vibrations.

Ion getter pump 56 is connected to and controlled by a control 59.Control 59 operates ion getter pump 56 in a manner that ion getter pump56 is switched-off at any time at which the gas field ion source isoperated and/or noble gas is supplied to the inner gas confining vessel41.

The ion getter pump 56 evacuating the mid column region 70 is attachedto the mid column region via a flange 72. In flange 72 a valve 57 isprovided which can be closed if the ion getter pump 56 needs to beexchanged or otherwise serviced or if the ion getter pump is switchedoff or if the ion getter pump should not evacuate the mid column region70. In this manner, exchange or servicing of ion getter pump 56 ispossible without venting the mid column region 70.

Ion getter pump 56 comprises a heater 58 which also is connected to andcontrolled by control 59. By the heater 58, the ion getter pump 56 canbe heated to release noble gas and other adsorbates from the ion getterpump 56 to clean it.

The outer gas containment 81 comprises a pressure measuring device 82which also is connected to control 59. The control 59 is configured, forexample by a computer with a software program, that only switches iongetter pump 56 on if the pressure within the outer gas containment 81 isbelow a predefined pressure value, i.e. when an output signal ofpressure measuring device 82 indicates a pressure in the outer gascontainment 81 below the predefined pressure value. In this manner thelife time of ion getter pump 56 can be extended.

As already described above in connection with FIG. 3, within the outergas containment 81 the gas field ion source is arranged. In FIG. 4 onlythat components of the gas field ion source are shown which form theinner gas confining vessel 41, i.e the base platform 31, the outertubular insulator 37, and the extractor electrode 38 with the extractorhole 39. Also shown in FIG. 4 is the getter 45 within the inner gasconfining vessel 41.

Also shown in FIG. 4 is the flapper valve 42 with its drive 43 whichalso is connected to and controlled by control 59. Flapper valve 42 canbe opened by its drive 43 if a quick evacuation of the inner gasconfining vessel 41 is desired, for example if a change of operation ofthe gas field ion source between operation with helium to generate ahelium ion beam and operation with neon to generated a neon ion beam isdesired.

The gas field ion microscope comprises a cooling device, for example adewar 52 with which the emitter tip as well as gas supply tube 40 andthe base platform 31 are cooled. The dewar 52 is thermally connected tothe components to be cooled like the base platform 31 or the gas supplytube 40. The dewar 52 comprises a vacuum jacket to insulate the innerchamber of the dewar configured to be filled with a cryogen from theouter world. Via a dewar jacket valve and a vacuum line the dewar jacketis connected to the sample chamber 10. In this way the vacuum in thevacuum jacket can be maintained at the pressure of the sample chamber.The dewar jacket valve can be closed if any process gases are suppliedto a sample positioned in the sample chamber, if the chamber is vented,or generally whenever the chamber pressure is above a predefinedpressure value, of for example 10⁻⁶ torr. By closing the dewar jacketvalve accumulation of condensible gases in the dewar jacket can beavoided.

The gas supply system of the gas field ion beam system shown in FIG. 4comprises two gas bottles 61, 62, one comprising helium and onecomprising neon. Both gas bottles have a pressure regulator to ensure aconstant gas pressure in the gas supply line after the pressureregulator. Following in both gas supply lines after the pressureregulators each gas supply line comprises a leak valve 63, 64. The leakvalves 63, 64 ensure a constant gas flow of the respective noble gasfrom the gas bottle 61, 62 to the tube 40, and accordingly into theinner gas confining vessel 41.

In the direction of gas flow from the gas bottles 61, 62 to the tube 40both gas supply lines are connected. Following in the direction of gasflow, in the combined gas supply line a purifier 65 and a gas valve 68follow before the gas supply line is connected to tube 40 whichterminates in the inner gas confining vessel 41.

The gas supply line comprises a bypass line 66 with a bypass valve 67 todirectly connect the gas supply line with the vacuum chamber 10.

Furthermore a heater 73 a (see FIG. 3) is provided on the gas supplytube 40 with which the gas supply tube 40 can be heated.

When operating the gas field ion beam system for several days with highhelium or neon gas flows, the operation of the gas field ion source caninclude a step of allowing the cryo-pumping surfaces, i.e. the baseplatform 31, the gas supply tube 40, the extractor electrode 38, theinsulators 33, 37 and the emitter tip 34 to warm up briefly. As a resultof this warming-up the accumulated cryo-adsorbed atoms can be desorbedand then pumped away via the turbo-molecular pumps 17, 60. Also the gasdelivery tube 40 which supplies the noble gas like helium or neon gasesfrom the external gas supply bottles 61, 62 to the proximity of theemitter tip 34 can be cryogenically cooled. This serves to purify thesupplied gases by allowing impurities, such as H₂O, CO, CO₂, N₂, O₂,etc., to be cryo-pumped onto the tube's 40 surface. To clean the surfaceof the tube 40 of the gas supply, it can be periodically heated to ahigh temperature by heater 73 a similar as the other cryo-pumpingsurfaces by the heater 73 c of the dewar 52 to a temperature of at least100° C., more preferable to 150° C. or even 200° C., to allow theseaccumulated adsorbates to be released and pumped away via the turbopumps 60, 17.

The gas delivery tube 40 has an inner diameter that is between 1 mm and6 mm. The gas delivery tube 40 connects the external gas delivery systemthrough the walls of the external gas containment 81, all the way to theinternal gas confining vessel 41. The gas delivery tube 40 has a bypassvalve 67, to facilitate the exhausting of the desorbed gases. The bypassvalve 67 prevents the desorbed gases from being largely trapped in theinner gas confining vessel 41. The bypass valve 67 can be completelyexternal to the vacuum housing, or integrated into the inner gasconfining vessel 41.

It has turned out that it is advantageous to periodically clean theemitter tip of adsorbed adatoms by one of three techniques. One of thethree techniques is to periodically heat the emitter tip 34 whilekeeping the components forming the inner gas confining vessel 41 atcryogenic temperature, for example to a temperature of 300° C. or morefor a time of 1 minute or more. This heating of the emitter tip 34 cancause the accumulated adsorbed atoms to be thermally excited so thatthey desorb and transfer to less critical surrounding surfaces. Thosesurfaces, primarily the surface of the extractor electrode 38 beingcold, will hold the adatoms and reduce the likelihood of beingtransferred back to the emitter tip 34.

Alternatively, instead of heating the emitter tip 34 it is possible touse an intense light focused on the emitter tip to cause the accumulatedadsorbed atoms to be photo-desorbed and hence leave the emitter tip 34clean and suited for stable ion emission.

As a further alternative, it is possible to increase the voltagedifference between the emitter tip 34 and the extractor electrode 38 sothat the electric field causes the accumulated adatoms to be desorbed.For example, if the voltage difference between the emitter tip 34 andthe extractor electrode during operation of the gas field ion source isnominally 30 kV for neon emission, the field can be increased to 32 kV,more preferably to 35 kV or 40 kV, to cause the adsorbates to beremoved.

The needs for one of these three above described techniques can beappraised by observing the emission pattern and seeing the effect of theindividual adsorbates. Or the needs for one of these three techniquescan be appraised by observing any unstable emission from the tip 34 ofthe emitter.

Respective field ion microscopic images of the emitter tip are shown inFIGS. 11 a and 11 b. FIG. 11 a shows the central trimer emissionpattern. The trimer atoms are brightest, but emission from non-trimeratomic emission sites are also visible. Nominally, the gun tilt isadjusted so that one of these three central emitted beams is aimed downthe ion column. During ideal operation, the emission pattern is verystable and constant over time. However, due to non-ideal vacuumconditions, or gas purity, an undesired atom or molecule can be adsorbedonto the emitter as show as a larger bright spot in FIG. 11 b.

These emission patterns can be regularly monitored to look for changesfrom such adsorbed molecules or atoms. The undesired adatom can belocated on the trimer atoms, or on one of the non-trimer atoms, or at adifferent location. The effect of the adsorbate is that the emissioncurrent from the trimer will be reduced or increased while the adsorbatecontinues to reside there. Therefore, the techniques described can beapplied until the adsorbate is removed, and the emission pattern isresorted to the original and desired appearance.

As described above, small amounts of spurious gas atoms that arrive atthe emitter tip of the gas field ion source can cause the emitted beamto fluctuate up and down in intensity or diminish gradually andprogressively. These effects can be diminished by a gas manifold (or gasdelivery system) that is designed for the purpose and operationalprocedures that optimize performance. The gas delivery system includes abypass valve that allows the gas delivery lines to be evacuated as acleaning process in preparation to their use with helium or neon gas.The gas delivery hardware is prepared with materials and methods thatare well established for UHV service. The gas delivery system isequipped with integrated heaters that can heat the gas manifold to hightemperatures such as 150° C., 200° C. or even 400° C. for long periodsof time in the range of 8 hours, 12 hour, or even 16 hours to help todesorb any vacuum contaminants. During this heating time, a valve 68 inthe line to the inner gas confining vessel 41 is closed, and a bypassvalve 67 in a pipe 66 leading to the sample chamber 10 is opened. As aresult, the evolved gases are pumped away to the sample chamber 10 wheretheir impact is not significant. The baking process can be repeatedafter the gas manifold is vented to atmosphere (e.g. after a serviceactivity such as a bottle replacement, or a valve replacement) or whenthe level of emission stability needs to be improved. A chemicallyactive purifier 65 can also be incorporated as a part of the gasmanifold to reduce common impurities. The purifier can be operated hotat 100° C., 200° C. or even 300° C. or at room temperature or anydesired temperature by way of a dedicated heater for the purifier. Thepurifier's heater can be powered by DC power so that there is nointerference from the 60 Hz or 50 Hz magnetic fields. The gas manifoldalso can comprise a pressure gauge 58, to monitor the pressuredownstream from of the precision leak valves, but before the gas isdelivered to the inner gas confinement.

The inner gas confining vessel of the gas field ion source has a builtin valve, the “flapper valve” 42 that, when opened, connects the innergas confining vessel 41 with the outer gas containment 81 and allows thepumping speed of the volume of the inner gas confining vessel to beincreased from about 1 liter/sec (when the only opening is through theextractor hole 39) to 22 liter/sec when the additional valve is open.Use of this valve can help to achieve a low base pressure which can helpwith the stable neon emission. Use of this valve can also speed up thetime to purge one gas (e.g. helium) before switching to another gas(e.g. neon). The valve can be mounted directly to the inner gasconfining vessel, or it can be located more remotely. The valve can alsobe incorporated into the gas delivery line 40.

A cryogenic connection can be provided that also serves as a gasdelivery tube from the gas supply bottles to the inner gas confiningvessel. The benefit is that there are fewer connections to the inner gasconfining vessel, and for service the connection and disconnection iseasier. Another benefit is that the gas path is suitably cold to providecryo-pumping of any impurities in the helium or neon gas. Anotherbenefit is that the gas delivery tube will be suitably heated to desorbthe impurities when the dewar is heated.

The inner gas confining vessel can be both heated and cooled through aflexible thermal conductive element 32 (shown in FIG. 3). The terminalend of the flexible thermal conductive element is a heater 73 c mountedto a cryogenic cooler. When the dewar is filled with a cryogen it servesto keep the gas field ion source cool. When the dewar is not filled witha cryogen, the heater 73 c can be powered to heat both the dewar and thecomponents forming the inner gas confining vessel. This design isespecially favorable since the dewar and the inner gas confining vesselare thermally intimate and it is not a simple matter to achieve atemperature difference between them. During the baking of both of theseparts, the power is about 25 watts, and the achieved temperature is 130°C. on the dewar, and 110° C. of the components forming the inner gasconfining vessel 41.

To reduce charging artifacts in images due to charging of the sample, aflood gun providing an electron beam can be provided which allows arelatively high energy in a range larger than 1 keV, larger than 1.5keV, or even larger than 2 keV. Higher energies are desirable for manysamples to better mitigate charging artifacts.

As a way of reducing the effect of vibration on the image quality, oneor more turbo molecular pumps are replaced by ion getter pumps. Theturbo molecular pumps are generally expensive. And due to their internalrotating parts, the turbo-molecular parts tend to impart vibrations tothe charged particle beam system and degrade the image quality. One wayto reduce costs and to eliminate turbo-vibrations, is to replace one ormore of them in favor of a getter ion pump (aka ion pump). The getterion pumps (or ion getter pumps) rely upon two pumping mechanisms. Thefirst method is chemical gettering to pump chemically active species.The second method is to directly bury atoms. The second method works forany gas molecules including noble gas atoms while the first method doesnot work for noble gas atoms because they are chemically inert. Thegettering effect is achieved by bonding of an active species to areactive material which commonly is a combination of titanium ortantalum and that is freshly evaporated by the getter ion pump. Thedirect burial is achieved by ionizing the molecule (by electron impact)and accelerating the resulting ion with a large electric field to anenergy of 3 keV or 5 keV, or 10 keV. The ion then strikes an adjacentsurface (titanium or tantalum) and is implanted into it to a typicaldepth of 10 to 100 nm. Upon burial, the gas species is no longeravailable to return to the vacuum vessel. Accompanied by the directburial is a sputtering effect in which chemically un-reacted titanium ortantalum molecules are sputtered away to become available for subsequentchemical gettering. The ion pumps however are known to be of limitedpumping speed for noble gases such as helium and neon because (1) theyare chemically relatively inert and thus are most effectively pumped bydirect burial, and (2) they are not easily ionized owing to their highionization energies, and (3) they can gradually diffuse out of theirburied states owing to their mobility, and the progressive erosion ofthe surface. To overcome the draw-back of limited life times of iongetter pumps in a noble gas environment they can be switched off whenthe gases present are primarily noble gases, such as when the gas fieldion source is operating. Alternatively, the getter ion pump can work inconjunction with a turbo pump wherein, the ion getter pump evacuatesonly a small intermediate vacuum space in the charged particle beamcolumn and the gas load to the ion getter pump is limited by adiaphragm.

When baking out the gas field ion source to attain the desired vacuumlevels, it is useful to follow a specific time ordering as describeswith reference to FIG. 6. In a first step 610 the external vacuumhousing, and the inner gas confining vessel 41, and the emitter tip ofthe gas field ion source all are heated to a high temperature of atleast 100° C., more preferable 150° C. or even 200° C. This heating cantake place concurrently for all components. However when the heatingprocess is completed at first in a step 611 the external vacuum housingis allowed to cool to room temperature. During the time when theexternal vacuum vessel cools down, the inner components such as theinner gas confining vessel 41 and the emitter tip of the gas field ionsource are continued to be heated. Then, after the external vacuumhousing has cooled down to room temperature in a step 612 the componentsforming the inner gas confining vessel are cooled to cryogenictemperatures while the heating of the tip 34 of the gas field ion sourceis still continued. Then, at a final step 613, after the componentsforming the inner gas confining vessel have been cooled to a cryogenictemperature, the heating of the ion emitter is discontinued so that thetip 34 of the gas field ion source is maintained to a cryogenictemperature. Other temperature versus time schemes can cause the tip ofthe gas field ion source to adsorb materials as they desorb from thesurrounding surfaces.

Control 59 can be configured, for example by a respective software code,to control the various heaters 73 a, 73 b, 73 c, 58 and the heatingcurrent through the supply lines of the emitter tip to ensure the aboveheating and cooling scheme.

Beam landing errors which can be evident as image vibrations can bereduced for example by eliminating the time varying magnetic andelectric fields that cause the ion beam to land in the wrong location.Generally electron and ion microscopes are powered by the standard 60 Hzand 50 Hz electric power systems. These “AC” power sources inadvertentlycreate small ripple voltages on the beam controlling electronics andthese can cause undesired beam landing errors. For example, 5 mV of 60ripple on the beam steering electrode will cause the beam to miss itsdesired target in a time varying way. Alternatively, the “AC” powersources can produce magnetic fields that can exert a force directly onthe charged particle beam, giving rise again to time-varying landingerrors. For example a 50 Hz magnetic field at amplitude of 5 milligausscan cause a time varying landing error of more than 1 nm. Commonly,these “AC” power sources provide power to the individual components thatcomprise and support the microscope. Examples include the turbo-pumps,the ion pumps, the vacuum gauges, the heaters, the mechanical stagemotors, the high voltage power supplies, the filament heater,pico-ammeters, chamber illuminators, detectors, electron flood gun, thecamera, DC low voltage power supplies, etc. Most of these systems arenot available except with AC power inputs. In other words the equivalentDC powered equivalents are generally not commercially available.However, it has shown to be desirable to design the gas field ionmicroscope with no AC powered components sources (50 Hz or 60 Hz) within3 meters of the microscope. This can be achieved by two methods: First,all components that are located within 3 meters of the microscope can bedesigned, specified, or modified to operate with only DC electricalpower, or pneumatic actuators. Second, the few items that involved ACpower with no alternatives available (e.g. the DC power supplies) can belocated remote from the microscope by at least 2 meters, more preferablemore than 3 meters. For example, the gas field ion microscope can havethe high voltages generated locally by DC to DC transformers. Someheating elements are operated by DC power. And some AC heaters can beused if they can be shut off when operating the microscope. Thecustomers can choose to located the operator console (with its own ACpowered computer and monitor) near or far to the microscope as theyprefer.

The sample stage 20 within the sample chamber 10 has a 5-axis, motorcontrolled stage with high repeatability (less than 2 microns), lowdrift (less than 10 nm/minute), and low vibration (<1 nm). The stageaxes are (in order from chamber's mounting surface 19 to the sample):Tilt, X, Y, Rotation, and Z. The tilt axis can tilt the sample from alimit of −5 degrees to 0 degrees (where the gas field ion source beamstrikes it orthogonally) to +54 degrees (where a gallium beam strikesthe sample orthogonally) to a limit of +56 degrees. To achieve thislarge tilt range with all the weight of all the superior axes, involvesa substantial torque with minimal net force. This tilt axis is drivenwith a conventional DC or stepper motor external to the vacuum, with ahermetic rotary feedthrough. All superior axes are actuated by piezoceramic actuators that provide very high stiffness (to reduce vibration)and an inherent breaking when not powered.

The gas field ion source is tilt-able by a motorized mechanism as shownin FIG. 5. The mechanism is designed so that when the motion iscomplete, vibrations are reduced by disengaging the drive mechanism. Byway of explanation, the gas field ion source can be tilted by smallangles (typically 1, 2, or 3 degrees in X and Y directions) to align theion source with respect to the column. In part, this tilt is involvedwhen the exact shape of the emitter is not readily controlled. In part,this tilt can be involved because usually three ion beams emanate fromthe apex of the emitter with an angular separation of about 1 degree.One of the emanating ion beams can be aimed down the axis of the ioncolumn for best performance. The tilting of the gas field ion sourceallows for this aiming of the chosen ion beam. As described above andalso shown in FIG. 5 the housing of the gas field ion source comprisestwo parts, an upper part 15 and a lower part 16, The upper part 15 ofthe housing is constrained to tilt by way of a concave spherical surfacethat mates with a corresponding convex spherical surface on the fixedlower portion 16 of the housing. The central point of the sphericalsurface is arranged so that it is coincident with the position of theapex of the emitter tip, thus providing a tilt motion that is concentricwith the apex of the emitter tip. The interface of the upper and lowerspherical surfaces provide sufficient friction to make the two piecesmechanically quite rigid and free from any measureable relativevibration.

In the system as shown in FIG. 5, the tilt of the upper housing 15relative to the lower housing 16 is achieved with a motorized tiltmechanism. The tilt drive mechanism is achieved by a fixed gantry 701fixed to the lower housing 16 that moves a peg that fits within areceptacle in the upper portion 15 of the housing. As the peg is movedby two orthogonal axes of motors 702, 703 (for X- and Y-tilts), it makescontact with the edge of the receptacle and causes the upper housing 15to move in the desired direction. This relative tilting of the twospherical surfaces is enabled again by the actuation of an air bearing.After the desired tilt is achieved, and the air bearing is disabled, thepeg is moved in a retreating direction so that it is no longer incontact with the edge of the receptacle. In this way, the motorized axes(and the vibrations they may introduce) are completely disengaged whenthe motion is no longer desired. Thus, the motors 702, 703 provide thetilting effect when desired, but are disengaged when their service iscomplete. Also it is worth noting that the upper housing 15 (the tilingpart) is equipped with a inclinometer 705 that provides a precisemeasurement of the tilt of the upper housing 15 relative to direction ofthe gravitational force. The inclinometer provides the tilt angle in twodirections (in X and Y direction) to the operator and to the controlthat controls the gun tilt motors. This allows the tilt of the upperhousing, and accordingly the ion gun tilt to be repeatedly moved fromone position to another and back again. The inclinometer 705 alsoprevents excessive tilt angles (e.g. +3 degree in X and +3 degrees in Y)that could damage the internal parts (which may be limited to just 4degrees total tilt from vertical). Also, it allows the upper housing 15to be restored to a standard tilt angle when it is desirable to executethe periodic source maintenance which relies upon the fixed cameravantage and fixed electrical contacts.

The process of adjusting the tilt of the upper housing is described withreference to FIG. 7. In a first step 801 the actual adjusted tiltposition of the upper housing 15 relative to the direction of gravity isstored by reading out an actual measurement value provided by theinclinometer 705. In the next step 802 the control 59 switches on theair supply for the air bearing between the two spherical surfacesbetween the upper housing 15 and the lower housing 16. Thereafter a step803 follows in which the tilt drives 702; 703 are activated whilecontinuing to read the actual measurement values provided by theinclinometer 705, until the inclinometer 705 provides the desired outputreading of the newly adjusted tilt position of the upper housing 15relative to the lower housing 16. When the new position is reached, in astep 804 the air supply for the air bearing between the upper housing 15and the lower housing 16 is stopped. In a step 805 the drives 702, 703are controlled to move into the opposite direction compared to themovement to reach the new tilt position until the peg disengages withthe receptacle. Thereafter the gas field ion beam system can be operatedin a step 806 with the upper housing 15 being in a new tilt positionrelative to the lower housing.

Since the old tilt position is stored, if desired the old tilt positioncan be readjusted by performing the above process anew but with oppositedirections of movement of the drives 702, 703 until the inclinometer 705provides the output signal indicating that the old tilt position hasbeen reached again. This process can be used when rebuilding the tip ofthe gas field ion source, where usually the rebuilding of the tip isperformed under a different orientation of the emitter tip than whenoperating the system to record images of a sample or process a sample.

By way of explanation, the gas field ion beam system can produce imagesof samples by detecting particles leaving the sample due to theimpinging ion beam, or manipulate and alter these samples withsub-nanometer precision. Therefore, it is critical that the ionmicroscope can operate without errors in the intended landing positionof the focused ion beam. Such landing errors can be quite small (e.g.smaller than 100 nm, smaller than 10 nm, or even smaller than 1 nm) andstill adversely impact the operation of the instrument. For the stableoperation of the gas field ion beam system, a proper amount of the noblegas for the gas ionization in the vicinity of the emitter tip is to beensured. To ensure a proper noble gas pressure, the gas supply systemincludes a leak valve 63, 64 (shown in FIG. 4 and in details in FIG. 8)that is either manually adjusted by the operator for the desired flowlevel, or adjusted by a motorized control system. In either case, theadjustment is established based upon a pre-established table of valuesthat relates the mechanical adjustment (e.g. manual turns of a knob 902or motor position) to the target value of the operating gas pressure. Asshown in FIG. 9 the gas pressure can be evaluated from a gauge 82located in the outer gas confinement 81 or a gauge 69 located in the gasmanifold (gas delivery system, shown in FIG. 4). And in either case, theadjustments can be deferred until the microscopes highest precisionactivities are completed. In other words, the normal control loop can beinterrupted during precision work. For example, if the gas pressurecrosses out of the acceptable range, the control 59 can provide anindication 906 to the operator via the computer interface (e.g. amessage indicating “gas pressure not in target”), or a light indicatorthat can change from green to red. And the operator can decide if thepresent microscope activity permits the corrective action, or if thisaction should be deferred. For reference, the normal gas pressure in theoperating microscope might be indicated by a pressure gauge 82 thatmight read from 2.0×10⁻⁶ Torr to 2.1×10⁻⁶ Torr. If the pressure crossesoutside of this range, it could affect the uniformity or consistency ofthe process which is underway. However, a corrective action could moreseriously affect the fidelity of the work that is underway. Accordingly,as shown in FIG. 9 the control 59 is configured that it only activates amotor 904 in a step 908 after the control has received a userinteraction 907 confirming that a correction of the gas flow throughneedle valve 63 or 64 is desirable at that time.

The leak valve that is in place can be a commercial manual precisionleak valve that is incorporated into the gas field ion beam system. Oneleak valve 63 can be provided for the helium gas delivery system and oneleak valve 64 can be provided for the neon gas delivery system asdisclosed above in reference to FIG. 3. These leak valves 63, 64 are tobe actuated manually without further modification, and with acalibration table that lists common desired gas pressures with thecorresponding knob turns to achieve these pressures. An alternativeembodiment for a motorized leak valve is shown in FIG. 8. This motorizedleak valve is based on a commercially available manual leak valve 63,64. At a housing portion 903 of the leak valve a drive motor 904 with aspindle 905 is attached. The spindle 905 acts on the manual adjustmentknob 902 of the manual leak valve.

Alternatively, the knob mechanism of the manual leak valve can bedispensed with entirely, and can be substituted by a piezo-ceramicactuator. Or furthermore alternatively, the knob mechanism of the manualleak valve can be dispensed with, and can be substituted by a cam-typedrive mechanism.

FIG. 10 shows the electrical set-up of an embodiment of gas field ionbeam system. As described above, the charged particle beam systemcomprises a charged particle source with an ion emitter having anelectrically conductive tip 34, an extractor electrode 38 and adeceleration or acceleration electrode 110. Following downward in thedirection of beam propagation follows a beam deflection system 112 withwhich the ion beam can be deflected in a direction perpendicular to itsdirection of propagation to scan the ion beam across a surface of asample to be positioned on sample stage 20. In addition the chargedparticle beam system comprises an objective lens comprising severalelectrodes 107, 108, 109 to focus the ion beam on the surface of thesample to be positioned on the stage 20.

For positioning a sample relative to optical axis 125 defined by thesymmetry of the electrodes 107, 108, 109 of the objective lens thesample stage 20 can be moved along and/or around several axis. Typicallya sample stage 20 has four or five axis of freedom for movement. Thesefive axis normally are linear movements perpendicular to optical axis125, linear movement along optical axis 125, tilt or rotation around anaxis perpendicular to the optical axis 125 and rotation around opticalaxis 125. For driving the movement a respective number of motor drivesare arranged at stage 20 of which two drives 105, 106 are shown in FIG.10.

In addition to the electrical motors 105, 106 the system comprises anumber of additional electrically driven components such as the actuator115 for the leak valves, the actuator 116 for the flapper valve, vacuumpumps 17, ion getter pump 56, heaters 73 a, 73 b, 73 c, etc. Forproviding the supply power for all these drives which may need to beoperated during operation of the charged particle beam system all theseelectrically powered devices are powered by the output power of an AC-DCconverter 114 which is itself powered by the normal 50 Hz or 60 Hz powersupply 113. This AC to DC converter 114 is configured to be positionedsome meters, e.g. at least two meters, away from the nearest ion opticalcomponent of the charged particle beam system. Accordingly, allelectrically driven components which are directly mounted in or at thecharged particle beam system and which, during conventional operation ofthe charged particle beam system may be operated, are configured to bepowered by the DC output of the AC-DC converter 114. In addition, forgenerating the high voltages to be applied to the emitter tip 34, theextractor electrode 38, acceleration and deceleration electrode 110,lens electrodes 107, 108 and deflection system 112 a DC to DC voltageconverter 118 is provided which is configured to generate severaldifferent high voltages from an in-coming DC voltage output of the AC-DCconverter 114. The various output signals of DC to DC converter 118 arelead to the respective electrode of the charged particle beam system byrespective supply cables or electrical supply lines 119-124.

By the above described electrical concept which avoids electricaldevices close to the charged particle beam system which are driven by ACvoltages and which need to be operated during operation of the chargedparticle beam system, disturbances with the frequency of the AC supplypower of 50 Hz or 60 Hz can be reduced to a large extent.

FIG. 12 shows a sectional view of a gas field ion source with aradiation shield 803. The design of this gas field ion source is verysimilar to the gas field ion source described above with reference toFIG. 3. Also in this case the source comprises an inner cylindricalisolator 33 holding the emitter tip 34 as well as an outer cylindricalisolator 37 surrounding the inner isolator 33 and holding the extractorelectrode 38 with hole 39. The space between the outer vacuum wall 801and the outer cylindrical electrode 37 form the outer gas containment 81and the space surrounded by the outer cylindrical electrode forms theinner gas confining vessel 41. To minimize radiative heat transfer fromthe components at room temperature such as the outer vacuum wall 801 theouter cylindrical electrode 37 and the extractor electrode 38 aresurrounded by a radiation shield 803. The radiation shield 803 generallycan be can shaped with a cylindrical tube with a cover 810 and a base812 at the base sides of the cylinder. The radiation shield can beplated with polished gold to minimize its radiation absorption. Theradiation shield is attached to the base plate 801 so that also theradiation shield is cooled to cryogenic temperature. Alternatively, theradiation shield also can have its own dedicated cooling connection tothe cooling system like a dewar. However a can shaped radiation shieldis less practical in cases in which a gas transfer is desirable betweena region surrounded by the radiation shield and a region outside theradiation shield.

FIGS. 13 a and 13 b show sectional views of heat shields which canaccomplish gas exchange between the region within the radiation shieldand outside the radiation shield. In FIG. 13 a the radiation shieldcomprises two concentric cylinders 805, 806, both made of metal withhighly radiation reflecting outer surfaces. Both cylinders comprise aplurality of slots 807, 808, wherein the slots 807 in the inner cylinder806 are rotationally offset with respect to the slots 808 in the outercylinder 805. The width of the slots, the distance between bothcylinders 805, 806 and the offset-angle between the slots in bothcylinders are selected in a manner that there is no direct line of sightfrom outside the outer cylinder 805 to the inside of the inner cylinder806. Any radiation passing a slot of the outer cylinder 805 therebyimpinges on a remaining material portion of the inner cylinder 806.

The embodiment in FIG. 13 b comprises a plurality of slabs 809 arrangedin a cylindrical fashion with each of the slabs being inclined at anangle unequal to 0° and 90° to a radial direction from a cylinder axis811. Also in this embodiment there is nearly no, or only a minimumdirect line of sight for radiation coming from outside the cylindricalregion to the inside of the cylindrical region surrounded by the slabs809.

In both embodiments in FIGS. 13 a and 13 b gas can flow through theslots 807, 808 or between the slabs 809 from inside the heat shield tooutside the heat shield or into the opposite direction while radiativeheat transfer from outside the region surrounded by the heat shield intothe region surrounded by the heat shield is strongly reduced.

The above disclosure can be summarized as follows:

-   -   In some embodiments of a process of operating a gas field ion        source the gas field ion source can be initially heated to        desorb any undesired atoms and molecules before any voltage is        applied to the ion source. Since the gas field ion source in        operation is cooled to cryogenic temperature (e.g. less than 90        Kelvin) this removes a large amount of atoms and molecules. The        heating can be brief, for example just for a few seconds, but it        should be a temperature of several hundreds of Kelvin, for        example 500 Kelvin or even more.    -   In some embodiments of a process of operating a gas field ion        source the gas field ion source can be operated at a maximum        tolerable voltage applied between the emitter tip and the        extractor electrode at times when it is not required to operate        the source at its optimum operating voltage. This “stand-bye”        voltage typically can be just below the voltage causing field        evaporation of the emitter tip. This can serve to maximize a        polarization of adsorbed atoms and hence minimizes the mobility        of the adatoms and thereby reduces the chance of the adatoms of        migrating toward the apex of the emitter tip.    -   In some embodiments the gas field ion source can be most        vulnerable to the effects of undesired atoms, so it can be        surrounded by cryogenically cooled surfaces so as to minimize        the probability of thermal desorption of any adsorbed atoms        which otherwise could arrive at the emitter of the gas field ion        source.    -   In some embodiments of a process of operating a gas field ion        source the cryogenically cooled surfaces can be periodically        heated or photostimulated to desorb adsorbed atoms or molecules.        During the heating process the gas field ion source should be        shut down by strongly reducing the voltage between the emitter        tip and the extractor electrode.    -   In some embodiments of a process of operating a gas field ion        source as a preparation step, the vacuum vessel and the gas        delivery system can be heated to high temperatures to facilitate        outgassing, and help to mobilize surface and bulk contaminants.        This can be done under partial vacuum in conjunction with other        volatilizing gases.    -   In some embodiments of a process of operating a gas field ion        source as a preparation step, the vacuum vessel and the gas        delivery system can be electropolished to minimize its surface        area.    -   In some embodiments of a gas field ion source the gun region can        be equipped with a chemical getter, for example commercially        available SAES getters, to provide high pumping of undesired gas        species. This chemical getters are very effective when the        desired gas species is a noble gas, since noble gases are not        pumped. The chemical getters also are very effective for pumping        hydrogen because this gas species is not effectively pumped by        cryogenic methods. When the getter is chemically activated, the        gas field ion source is normally heated and the gas field ion        source can be disabled.    -   In some embodiments of a gas field ion source the gas delivery        tube can pass through a cryogenic trap to cause impurities to        condense. In some embodiments such portion of the gas delivery        system can have a valve to permit purging.    -   In some embodiments of a gas field ion source the gas delivery        system can have a purifier that contains a heated or unheated        chemical getter to chemically trap any undesired atoms or        molecules.    -   In some embodiments of a gas field ion source the gas delivery        system can have a bye-pass so that the contents, including the        undesired atoms and molecules, can be purged into a vessel other        than the ultimate gun region.    -   In some embodiments of a gas field ion source the region of the        gas field ion source can be equipped with an ion pump to pump        undesired gas atoms, and the ion pump can be disabled when the        desired gas is delivered and enabled during standby of the gas        supply.    -   In some embodiments of a gas field ion source the vacuum vessel        can be equipped with a conformal coating of non-evaporable        getter similar to SAES getters.    -   In some embodiments of a gas field ion source the vacuum vessel        can be equipped with a hydrogen pumping getter such as a        titanium sublimation pump.    -   In some embodiments of a process of operating a gas field ion        source the gas field ion source can be operated for a period of        time to help condition or prepare the surfaces. Under the        conditioning period, the ion source can be purged of adsorbed        gas atoms through the bombardment of energetic, highly polarized        neutral atoms. Also during the conditioning period, the        extractor electrode, the suppressor electrode, lens electrodes        and other surfaces, onto which the ion beam can impinge during        operation of the gas field ion source, can be cleaned of        adsorbed atoms or chemically attached atoms. The conditioning        steps can be carried out with a heavier gas species if desired        to accelerate the process.

In the above description features of aspects of different inventions aredisclosed in combination. The scope of the present invention is notintended to be restricted to such combinations of features but has to beunderstood to be solely defined by the following claims.

What is claimed is:
 1. A gas field ion source, comprising: a gunhousing, an electrically conductive gun can base attached to the gunhousing, an inner tube mounted to the gun can base, the inner tubecomprising an electrically isolating material, an electricallyconductive tip attached to the inner tube, an outer tube mounted to thegun can base, the outer tube comprising an electrically isolatingmaterial, and an extractor electrode attached to the outer tube, theextractor electrode having an opening for the passage of ions generatedin proximity to the electrically conductive tip.
 2. The gas field ionsource of claim 1, further comprising a gas supply comprising aterminating tube attached to the gun can base.
 3. The gas field ionsource of claim 2, wherein the gas supply is configured to supply afirst gas in a first mode of operation of the gas field ion source, thegas supply is configured to supply a second gas in a second mode ofoperation, and the first gas is different from the second gas.
 4. Thegas field ion source of claim 2, further comprising a vacuum pumpoperatively connected to the outer housing, wherein the vacuum pump isconfigured to evacuate gas out of the outer housing.
 5. The gas fieldion source of claim 1, further comprising a thermal conductor connectedto gun can base, wherein the thermal conductor is thermally connected toa cooling device.
 6. The gas field ion source of claim 5, wherein thecooling device comprises a dewar.
 7. The gas field ion source of claim1, further comprising a heater within the inner tube, wherein the heateris electrically connected to the electrically conductive tip.
 8. The gasfield ion source of claim 1, further comprising a flapper valve arrangedat the gun can base, wherein the flapper valve is configured to increasea gas flow from a region within the outer tube into a region surroundingthe outer tube.
 9. The gas field ion source of claim 1, wherein a volumeenclosed by the gun can base, the outer tube and the extractor electrodeis smaller than 60 cm³.
 10. The gas field ion source of claim 1, furthercomprising: a first high voltage supply electrically connected to theelectrically conductive tip, and a second high voltage sourceelectrically connected to the extractor electrode.
 11. The gas field ionsource of claim 1, further comprising a heat shield configured to reduceradiative heat transfer from the gun housing to a volume surrounded bythe outer tube.
 12. A gas field ion source, comprising: a gun housing,an electrically conductive gun can base attached to the gun housing, aninner tube mounted to the gun can base, the inner tube comprising anelectrically isolating material, an electrically conductive tip attachedto the inner tube, an outer tube mounted to the gun can base, the outertube comprising an electrically isolating material, an extractorelectrode attached to the outer tube, the extractor electrode having anopening for the passage of ions generated in proximity to theelectrically conductive tip, a gas supply comprising a terminating tubeattached to the gun can base, and a thermal conductor connected to guncan base, wherein the thermal conductor is thermally connected to acooling device.
 13. The gas field ion source of claim 12, wherein thecooling device comprises a dewar.
 14. The gas field ion source of claim12, further comprising a heater within the inner tube, wherein theheater is electrically connected to the electrically conductive tip. 15.The gas field ion source of claim 12, further comprising a flapper valvearranged at the gun can base, wherein the flapper valve is configured toincrease a gas flow from a region within the outer tube into a regionsurrounding the outer tube.
 16. The gas field ion source of claim 12,wherein a volume enclosed by the gun can base, the outer tube and theextractor electrode is smaller than 60 cm³.
 17. A gas field ion source,comprising: a gun housing, an electrically conductive gun can baseattached to the gun housing, an inner tube mounted to the gun can base,the inner tube comprising an electrically isolating material, anelectrically conductive tip attached to the inner tube, an outer tubemounted to the gun can base, the outer tube comprising an electricallyisolating material, an extractor electrode attached to the outer tube,the extractor electrode having an opening for the passage of ionsgenerated in proximity to the electrically conductive tip, and a gassupply comprising a terminating tube attached to the gun can base,wherein the gas supply is configured to supply a first gas in a firstmode of operation of the gas field ion source, the gas supply isconfigured to supply a second gas in a second mode of operation, and thefirst gas is different from the second gas.
 18. The gas field ion sourceof claim 17, further comprising a vacuum pump operatively connected tothe outer housing, wherein the vacuum pump is configured to evacuate gasout of the outer housing.